1. Field of the Invention
This invention relates to the fluidized catalytic cracking (FCC) conversion of heavy hydrocarbons into light hydrocarbons with a fluidized stream of catalyst particles. More specifically, this invention relates to an FCC process for the production of light olefins.
2. Description of the Prior Art
Catalytic cracking is accomplished by contacting hydrocarbons in a reaction zone with a catalyst composed of finely divided particulate material. The reaction in catalytic cracking, as opposed to hydrocracking, is carried out in the absence of added hydrogen or the consumption of hydrogen. As the cracking reaction proceeds, substantial amounts of coke are deposited on the catalyst. The catalyst is regenerated at high temperatures by burning coke from the catalyst in a regeneration zone. Coke-containing catalyst, referred to herein as xe2x80x9ccoked catalystxe2x80x9d, is continually transported from the reaction zone to the regeneration zone to be regenerated and replaced by essentially coke-free regenerated catalyst from the regeneration zone. Fluidization of the catalyst particles by various gaseous streams allows the transport of catalyst between the reaction zone and regeneration zone. Methods for cracking hydrocarbons in a fluidized stream of catalyst, transporting catalyst between reaction and regeneration zones, and combusting coke in the regenerator are well known by those skilled in the art of FCC processes.
Propylene is conventionally produced through FCC processes, dehydrogenation processes, and predominantly from steam cracking processes. The demand for propylene is projected to begin to outstrip supply. FCC units are filling some of this growing demand for propylene. Typically, however, FCC units produce only around 5 wt-% of propylene. Consequently, modifications to FCC units that can increase propylene production are necessary. Several references disclose modified FCC processes to improve propylene yields.
Many of these processes increase propylene yields by increasing conversion by utilizing longer reaction times and hot catalyst temperatures. One such process called deep catalytic cracking (xe2x80x9cDCCxe2x80x9d) requires 5-10 seconds of contact time to increase propylene yields. However, this process also yields a relatively substantial quantity of undesirable dry gas; i.e., hydrogen, ethane and methane. See David Hutchinson and Roger Hood, Catalytic Cracking to Maximize Light Olefins, PETROLE ET TECHNIQUES, March-April 1996, at 29. U.S. Pat. No. 4,980,053 also discloses a deep catalytic cracking process that cracks over a mixture of Y-type zeolite and a pentasil, shape-selective zeolite to give substantial yields of propylene. Similarly, this patent discloses an effort to prolong the catalyst contact time which is probably the reason for it reporting relatively high yields of dry gas.
Other patents disclosing short catalyst contact times do not recognize significant light olefin yields. U.S. Pat. No. 5,965,012 discloses an FCC process with a catalyst recycle arrangement with a very short contact time of the feed and catalyst. However, the short contact time does not take place in the riser. Cracking takes place in a chamber where regenerated and carbonized catalyst contacts the feed. The cracked products are immediately withdrawn from the cracking chamber and separated from the catalyst in a conduit which is separate from the riser. U.S. Pat. No. 6,010,618 discloses another FCC process which provides for very short catalyst-to-feed contact time in the riser by quickly removing cracked product from the riser well below halfway to the outlet of the riser. U.S. Pat. No. 5,296,131 discloses ultra-short FCC catalyst contact times to improve selectively to gasoline while decreasing coke and dry gas production. These patents do not target significant production of light olefins.
U.S. Pat. No. 5,389,232 discloses quenching the feed and catalyst mixture with naptha in the riser to shorten the catalyst-to-feed contact time to obtain light olefins. This patent, however, reports relatively low yields of light olefins.
Other patents disclose processes that use catalyst recycle without regeneration. U.S. Pat. No. 3,888,762 discloses sending stripped catalyst and regenerated catalyst to the base of the riser without mixing. U.S. Pat. No. 4,853,105 discloses an FCC process whereby stripped, coked catalyst is recycled to the riser just less than mid-way up the riser. This stripped, coked catalyst contacts feed in the riser for less than 1 second but the feed also has contact time with regenerated catalyst from about 10 to about 50 seconds. U.S. Pat. No. 5,858,207 discloses an FCC process wherein regenerated catalyst and stripped coked catalyst are subjected to secondary stripping before being returned to the riser to contact feed. U.S. Pat. No. 5,372,704 discloses an FCC process wherein spent catalyst from a first FCC unit is charged to a riser of a second naphtha cracking unit and then recycled back to the riser of the first FCC unit.
U.S. Pat. Nos. 4,990,314, 4,871,446, and 4,787,967 disclose two component catalyst FCC systems in which a portion of the catalyst is recycled back to the riser without regeneration. Specifically, one component of the catalyst typically includes a large-pore zeolite for cracking the larger molecular hydrocarbons and the second component includes a medium pore zeolite for cracking the smaller molecular hydrocarbons. These patents, by recognizing that the catalyst component with medium pore zeolite are susceptible to hydrothermal degradation, attempt to recycle a homogeneous composition of the catalyst component with medium pore zeolite back to the riser without undergoing regeneration. The exclusion of the catalyst component with medium pore zeolite from the regeneration zone requires either special configuration of the catalyst matrix and/or complex design of the apparatus. U.S. Pat. No. 4,717,466 discloses a variant of this process wherein two risers are utilized. One riser has a greater concentration of ZSM-5 catalyst component in a base with a larger diameter for prolonged contact time with a lighter feed.
PCT Publication WO 95/27019 reports aggregate yields of 26.2 wt-% of ethylene, propylene and butylene in a circulating fluidized bed reactor having a relatively short residence time of 0.1 to 3.0 seconds. The reaction zone disclosed in this application terminates at an external cyclone which separates catalyst and products. The catalyst is stripped and sent either to the base of the reaction zone or a circulating fluidized bed regenerator. This publication does not teach use of a medium or smaller pore zeolite in the catalyst composition.
Some do not use a medium to smaller pore zeolite in the catalyst composition, perhaps, for fear that the concentration of the larger pore or amorphous catalyst would be insufficient to crack the feed down to naptha range molecules. Two cracking steps have to take place to obtain light olefins. First, a catalyst component containing a large pore zeolite and/or an active amorphous material cracks the feed into naphtha range hydrocarbons. Second, a catalyst component containing a medium or small pore zeolite cracks the naphtha range hydrocarbons into light olefins. The medium or small pore zeolite cannot crack the large hydrocarbon molecules in the feed. Hence, the concern that a high concentration of the medium or small pore zeolite component in the catalyst composition could unduly dilute the amorphous or large pore catalytic component to restrain the first step of cracking FCC feed down to naphtha range hydrocarbons.
U.S. Pat. No. 6,106,697 avoids this concern by using a two-stage catalytic cracking system wherein a large pore zeolite component cracks the feed in an FCC unit down to naphtha range hydrocarbons which is then cracked over a medium to small pore zeolitic catalyst component in a second FCC unit to get light olefins. U.S. Pat. No. 5,997,728 discloses an FCC process that cracks feed over a catalyst composition containing relatively large proportions of medium or smaller pore zeolite catalyst and large pore zeolite for 5 seconds of catalyst contact time to obtain good yields of propylene but with high yields of dry gas. However, PCT Publication WO 00/31215 discloses a catalytic cracking process which uses a ZSM-5 and/or ZSM-11 zeolite component on a substantially inert matrix material in a catalytic cracking process to obtain high yields of light olefins
U.S. Pat. No. 5,597,537 teaches an FCC process that uses a high ratio of catalyst to feed and higher regenerator temperatures to ensure that gasoline fraction olefins will overcrack to provide a high yield of C3 and C4 olefins. This patent also teaches recycling part of the coked catalyst to a mixing chamber at the base of the riser while transporting another portion of the coked catalyst to the regenerator for regeneration. The regenerated catalyst portion and the recycled, coked catalyst portion are combined in a blending vessel and allowed to thermally equilibrate before being introduced to the riser to catalyze fresh feed. Although this patent does indicate that the disclosed process could be used for lower residence time cracking, it explains that lower residence times are desired to prevent catalyst from coking, not to yield higher quantities of C3 and C4 olefins. This patent also teaches adding a medium pore zeolite component to the catalyst composition in an effort to prevent catalyst coking. However, it does not couple the use of a medium pore zeolite component and short contact times to obtain greater yields of light olefins. This patent reflects the concern that a large pore zeolite and/or active amorphous containing catalyst component that is diluted with a medium pore zeolite component and coked from recycling without regeneration may not be sufficiently active to crack feed down to naptha range hydrocarbon. Hence, the desire to minimize coking.
It is an object of this invention to provide a FCC process that produces high yields of light olefins with less production of dry gas.
An FCC process is modified to produce greater yields of light olefins; particularly, ethylene, propylene and butylene with less production of dry gas; i.e., hydrogen, methane and ethane at relatively high conversion.
We have discovered that recycling coked catalyst including a large pore zeolite and/or an active amorphous material and a zeolite with no greater than medium average pore size and blending it with regenerated catalyst improves the yield of light olefins and the overall conversion. We have discovered this to be the case even at lower riser residence times. Additionally, the lower temperature of the catalyst resulting from blending hot regenerated catalyst and cooler recycled catalyst improve olefin selectivity.
Specifically, an embodiment of the present invention is a process for fluidized catalytic cracking of a hydrocarbon feed stream to obtain light olefins. The process comprises contacting the hydrocarbon feed stream with a blended catalyst comprising regenerated catalyst and coked catalyst. The catalyst has a composition including a first component comprising a large pore molecular sieve and/or an active amorphous material and a second component comprising a zeolite with no greater than medium pore size. The zeolite with no greater than medium pore size comprises at least 1.0 wt-% of the catalyst composition. The contacting of the catalyst in the feed stream occurs in a riser to crack hydrocarbons in the feed stream and obtain a cracked stream containing hydrocarbon products including light olefin and coked catalyst. The cracked stream is passed out of an end of the riser so that the hydrocarbon feed stream is in contact with the blended catalyst in the riser for less than or equal to 2 seconds on average. The hydrocarbon products including light olefins are separated from the coked catalyst. A first portion of the coked catalyst is passed to a regeneration zone wherein coke is combusted from the catalyst to produce regenerated catalyst. The regenerated catalyst has substantially the same relative proportions of the first component and the second component as the blended catalyst that contacts the hydrocarbon feed stream. The second portion of the coked catalyst is blended with the regenerated catalyst to make the blended catalyst. Then the blended catalyst is introduced to the riser.
In another embodiment, the catalyst composition may comprise up to 80 wt-% of the catalyst composition. In a further embodiment, the molecular sieve may be either an X-type or a Y-type zeolite.
In another embodiment, the second portion of the coked catalyst and the regenerated catalyst are blended outside of the riser before contacting the feed stream.
Other embodiments of the present invention include the partial pressure of the hydrocarbons in the riser being less than or equal to 172 kPa (25 psia); a diluent in the riser being greater than or equal to 10 wt-% of the feed stream; a ratio of catalyst to feed in the riser being greater than or equal to 10; a ratio of coked catalyst to regenerated catalyst in the riser being in the range of 0.3 to 3.0; a temperature of the cracked stream at the top end of the riser being in the range of 510xc2x0 to 621xc2x0 C. (950xc2x0 to 1150xc2x0 F.); a temperature of the blended catalyst being greater than or equal to 28xc2x0 C. (50xc2x0 F.) lower than the temperature of the regenerated catalyst coming from the regenerator; and the zeolite with no greater than medium pore size comprising at least 1.75 wt-% of the blended catalyst composition.
In another embodiment, the present invention comprises a process for fluidized catalytic cracking of a hydrocarbon feed stream to obtain light olefins. The process comprises contacting the hydrocarbon feed stream with a blended catalyst at an initial temperature of 621xc2x0 to 677xc2x0 C. (1150xc2x0 to 1250xc2x0 F.) in a reactor conduit to crack hydrocarbons in the feed stream and to obtain a cracked stream containing hydrocarbon products including light olefins and coked catalyst. The blended catalyst comprises regenerated catalyst and coked catalyst. The catalyst has a catalyst composition including a first component and a second component comprising a molecular sieve with no greater than medium average pore size. The cracked stream is passed out of the reactor conduit at a temperature of 538xc2x0 to 593xc2x0 C. (1000xc2x0 to 1100xc2x0 F.) such that the hydrocarbon feed stream is in contact with the blended catalyst in the riser for less than or equal to 2 seconds on average. The hydrocarbon products including light olefins are separated from the coked catalyst. The first portion of the coked catalyst is passed to a regeneration zone and coke is combusted from the catalyst to produce regenerated catalyst. The regenerated catalyst has substantially the same relative proportions of the first catalyst component and the second catalyst component as the blended catalyst that contacts the hydrocarbon feed stream. A second portion of the coked catalyst is blended with the regenerated catalyst and introduced as blended catalyst to the reactor conduit.
In another embodiment, the first component of catalyst comprises a large pore molecular sieve and/or an amorphous material.
In further embodiments, the partial pressure of the hydrocarbons in the riser may be less than or equal to 172 kPa (25 psia); a diluent in the riser may be greater than or equal to 10 wt-% of the feed stream; the ratio of catalyst to feed in the riser may be greater than or equal to 10; and the ratio of coked catalyst to regenerated catalyst in the riser may be in the range of 0.3 to 3.0.
Another embodiment of the present invention is a process for fluidized catalytic cracking of a hydrocarbon feed stream to obtain light olefins. The process comprises contacting the hydrocarbon feed stream with a catalyst composition including at least 1.0 wt-% of the zeolite having no greater than medium average pore size and at least 0.1 wt-% coke. The contacting occurs in a single reactor for no more than 2 seconds.
In a further embodiment, the hydrocarbon partial pressure may be less than 172 kPa (25 psia) and the catalyst composition may be at a temperature of about 621xc2x0 to 677xc2x0 C. (1150xc2x0 to 1250xc2x0 F.) before contacting the feed stream.
Additional objects, embodiments, and details of this invention will become apparent from the following detailed description.